1. Field of the Invention
The present invention relates to a radiation monitor, or more particularly, to a radiation monitor capable of monitoring a dose originating from a radiotherapy apparatus, a non-destructive inspection radiologic equipment, or other radiation generating apparatus without the influence of an ambient temperature of air pressure.
2. Description of the Related Art
FIG. 6 is a schematic diagram showing a radiation generating apparatus by the name of a medical linac or linear accelerator. In FIG. 6, a gantry 52 is installed to rotate against a stand 51 with a virtual rotation axis 53 as a center. A virtual point 54 referred to as an isocenter and thought to be a center of a therapeutic radiation is an intersection between the rotation axis 53 and a radiation center axis 21 to be described later.
Equipment for generating radiations are incorporated in the gantry 52. The equipment will be described in detail. That is to say, the gantry 52 accommodates an electron gun 55 for generating electrons, an accelerating tube 56 for accelerating electrons to produce a high energy, a vacuum beam duct 57 running through the accelerating tube 56 for routing accelerated electrons, and a deflecting electromagnet 58 for deflecting electrons. 59 is an orbit of the electrons. Also incorporated is a metallic target against which electrons collide to generate an X ray. For electron beam therapy, a scatterer for scattering electrons is installed at the position of the target 60 on behalf of the target 60.
Distribution of an X ray the target 60 generates is restricted by a primary collimator 61. A flattening filter 62 is installed to flatten the energy spectrum within the distribution of the X ray the target 60 generates. If a scatterer is used as the target 60 for electron beam therapy, a numeral 62 denotes a secondary scatterer. The intensity of an X ray or an electron beam is monitored by a radiation monitor 63.
Collimator blocks 64 and 65 control distribution of an X ray according to the size of a lesion to be treated. As shown in FIG. 6, the collimator block 64 is made up of a pair of blocks 64a and 64b. The collimator block 65 lies almost perpendicularly to the collimator block 64 and consists of a pair of blocks, which are not shown, similarly to the collimator block 64.
A virtual center axis is running perpendicularly to the target 60, which is referred to as a beam center 21. A radiation 20 represents the distribution of a radiation restricted by the primary collimator 61. A patient treatment table 66 is provided with a tabletop 67 on which a patient 68 lies.
FIG. 7 is a cross-sectional diagram showing a transmission type parallel plate chamber representing an example of a radiation monitor 63. A high-voltage electrode 2 serves as one of parallel plates, which is formed with a thin metallic or metal-deposited insulating sheet. A collecting electrode 3 collects ionized ions or electrons, which is made of the same material as the high-voltage electrode 2. A frame 4 is hollowed in the form of a column or prism, supporting and isolating the two electrodes 2 and 3. The electrodes 2 and 3, and frame 4 form a sealed ionization space 1 in which gas is ionized with a radiation.
The electrodes 2 and 3 are locked in the frame 4 with a bracket 5 and a set screws 7. A general earth electrode 28 serving as a thin metal cover is installed to protect the electrodes 2 and 3. A seal 6 is employed to shield a space formed with the metallic cover 28 and frame 4 from gas coming from an external space of a radiation monitor. A high-voltage connector 8 is installed to supply high voltage via an external circuit. Also installed is a collecting electrode connector 9 for providing the ions or electrons the collecting electrode 3 collects as ionization current.
FIG. 8 shows a transmission type parallel plate chamber representing other example of a conventional radiation monitor 63a. In the radiation monitor 63a of FIG. 8, unlike a radiation monitor 63 of FIG. 7, a frame 4 is formed as a rigid plate including electrodes 2 and 3. The volume of an ionization space 1 does not vary depending on an external temperature or an air pressure. The ionization space 1 is shielded from external gas.
FIG. 9 is a schematic diagram for explaining the relationship between a radiation monitor 63 or 63a and an external circuit. In FIG. 9, an electrode 2 is connected to a high-voltage power supply 71 via a high-voltage connector 8. An electrode 3 is connected to a current-voltage converter 72 for converting ionization current into voltage, an amplifier 73, a display 74 for indicating a dose, and a control system 75 for feeding back the operation of a medical linac according to a monitored dose via a collecting electrode connector 9.
Next, the operations will be described. In radiotherapy using the configuration of FIG. 6, a patient 68 is positioned by operating a treatment table 66 and a tabletop 67, so that the lesion will align with an isocenter 54.
As for a therapeutic radiation, electrons an electron gun 55 emits are accelerated by an accelerating tube 56 to yield a given level of energy. Then, the electrons are deflected by a deflecting electromagnet 58 to follow an orbit 59. Finally, the electrons hit a target 60.
As a result, an X ray develops from the target 60. The X ray is controlled by a primary collimator 61 to form a radiation 20. The radiation 20 represents an energy spectrum symmetrical with respect to a beam center axis 21. To meet therapeutic needs, the energy spectrum of the radiation 20 must be uniform, which, therefore, is flattened by a flattening filter 62.
In treatment planning for electron beam therapy, a scatterer for scattering an electron beam is installed at the position of the target 60 and a secondary scatterer is placed at the position of the flattening filter 62. Thus, an electron beam distribution becomes uniform over the regions of the radiation 20. Then, the radiation 20 irradiates a lesion of the patient 68. At this time, depending on the size of the lesion, a pair of collimator blocks 64 and 65 is used to align the electron beam with a given region.
For electron beam therapy, an applicator (not shown) may be employed to confine a passage of an electron beam from the collimator block 65 to a patient.
The aforesaid radiation generating mechanism is locked in a gantry 52. Then, the gantry 52 is rotated against a stand 51 around a rotation axis 53 so that the radiation 20 can be irradiated from around the body axis of a patient 68.
Radiotherapy is proceeded as described previously. A dose of a radiation 20 incident on a patient 68 must be monitored in real time. It is a radiation monitor 63 or 63a to detect the dose of the radiation 20.
FIG. 7 shows an example of a conventional radiation monitor 63. In FIG. 7, a high-voltage electrode 2 is opposing a collecting electrode 3 to form a so-called transmission type parallel plate chamber. The electrodes 2 and 3 run through a frame 4 to reach respective electrode connectors 8 and 9. A metallic cover 28 is fixed to the frame with a bracket 5 and a screw 7. Thus, the metallic cover 28 and a seal 6 form an airtight ionization space 1.
A radiation ionizes gas when passing through the air. High voltage which is high enough to move ionized ions or electrons toward an electrode is supplied to the high-voltage electrode 2. Then, the collecting electrode 3 is grounded through a low impedance. An electric field develops between the electrodes 2 and 3. Ions or electrons ionized by the radiation are attracted to counter electrodes. The collecting electrode 3 collects either the ions or electrons, so that ionization current can be monitored as a dose.
FIG. 9 shows the foregoing procedure. A high-voltage power supply 71 supplies high voltage to a high-voltage electrode 2 via a high-voltage connector 8. A collecting electrode 3 is grounded through a low input impedance of a current-voltage converter 72. Ionization current is converted into voltage by the current-voltage converter 72, then amplified by an amplifier 73 to have a given strength. Then, the amplified signal indicates a dose on a display 74 and serves as an input of a control system 75. At this time, the relationship between the ionization current and dose is represented as follows: EQU i=kD.times.PV/T (1)
where, i is ionization current, k, a proportional constant, D, a radiation intensity, P, an air pressure in an ionization space, T, an absolute temperature in the ionization space, and V, a volume of an ionized region.
The expression (1) means that ionization current faithfully represents a radiation intensity but is affected by an air pressure or temperature. Therefore, an airtight space is formed as shown in FIG. 7 in an effort to minimize the influence of an air pressure or temperature.
A radiation monitor 63a shown in FIG. 8 is devised to avoid the influence of an air pressure or temperature. A frame 4 is made of ceramic or other tough and light material, having an airtight space inside. A high-voltage electrode 2 and a collecting electrode 3 are arranged in the space to form a transmission type parallel plate chamber.
In the foregoing configuration, the volume of the internal airtight space does not vary regardless of an external air pressure or temperature of the frame 4. As far as the volume of a sealed space does not vary, the quotient of P/T in the expression (1) is constant. Therefore, the dose monitor provides a value of ionization current which is proportional to a dose regardless of an external air pressure or temperature.
In the aforesaid conventional radiation monitor of FIG. 7, a space formed with a metallic cover 28 and a frame 4 is airtight. However, since a value V in the expression (1) does vary in an ionization space 1, the quotient of P/T does not become constant. This means that the monitored value is affected by an external air pressure or temperature. In FIG. 8, when an X ray whose energy is low enough to be absorbed into ceramic or other light material is irradiated, the radiation monitor works effectively. However, in electron beam therapy, dose absorption of the radiation monitor itself is too large to be ignored. This cripples electron beam therapy.